p;ChT International supplement, 29 April I96S
Air-Cuihion Vehicle*
The fact that a correctly matched lift
system is able to accommodate quite
large changes in operating conditions
can give the impression that matching
is not important. This is not true. Only
a well designed system can operate
efficiently under different conditions;
a mis-matched system will be inefficient
and sensitive to changing conditions.
It is, however, still advantageous to have
the well designed system operating in
the planned range.
For the same pressure rise and volume
flow the blade tip speed Vb of an axial
fan is about twice the equivalent peripheral speed of a centrifugal fan.
Therefore the centrifugal fan can
produce high pressure rises without very
high rotational speed. In most cases
a centrifugal fan and associated ducting
will occupy more space than an axial
fan of the same volume flow. The
simple air jet craft requires higher
pressure rises and lower volume flows
than the equivalent plenum chamber
craft. The characteristics of centrifugal
fans are therefore suited to air jet craft.
Typical centrifugal fan performance
curves are shown in Fig 21. In the sets
of equations 28 and 29, Vb for a centrifugal fan is the tangential speed of the
periphery of the fan. In comparison
with the axial fan the operating values
of Hb and G are considerably higher.
The main reason for this is the fact that
the centrifugal fan is not limited by
stalling conditions.
Whatever the type of craft, lift
system, or fan the same method of
component matching can be used. The
same method applies whether the craft
is a 200-ton ferry or a 21b model; only
the numbers are different.
The fan is the most important single
item in the lift system. It converts the
power of the lift engine into air power
in the lift system. The efficiency of the
fan has a direct effect on the effectiveness of the lift system, and it is a component with little possibility of compromise. It has been shown above that it
must be correctly chosen for size and
run at the correct speed.
In addition to this the fan must be
designed to give the desired performance, as shown in Fig 20a. This involves
considerable calculation to determine
the geometry of the blading. The making of the blades is a difficult task
requiring accurate reproduction of
complicated shapes, and the fan
assembly must be designed and manufactured to ensure it remains in one
piece during operation. Consider the
axial fan required for the plerfum chamber craft used to illustrate design methods. It is 32in in diameter, rotating at
2,245 r.p.m., giving a tip speed of
FLOW
Fig 22: graph illustrating the effects of fan
stalling
312ft/sec or 213 m.p.h. If solid hardwood was used for the blades the centrifugal force on each blade would be close
to 1,0001b, or nearly half a ton. Probably six blades would be anchored to
the 13in diameter boss. The manufacture of such a fan would require knowledge, experience, skills and facilities
Fig 23: chart showing typical performance data of axial and centrifugal fans. Method of use: To find the diameter and rotational speed of
a fan to give a required pressure rise and volume flow, (7) select the pair of scales appropriate to the type of fan, (2) draw a horizontal line
from the value of pressure rise, (3) draw a vertical line from the value of volume flow, (4) estimate the fan diameter and speed given by the
intersection of the two lines just drawn. For example, an axial fan to deliver 600cu ftI sec at 20lb/sq ft would be 50in in diameter and rotate at
1,500 r.p.m. The chart may also be used for the reverse operation of finding the pressure rise and volume flow delivered by a fan of a given
diameter and rotational speed
O
I
too
300
200
Q AXIAL cu f t / s r c
4OO
500
6OO
I
x
-1
IOO
2OO
3OO
400
5OO
6OO
7OO
80O
900
I00O
O CENTRIFUGAL cu ft / sec
60